This invention relates generally to adhesives, and more particularly to an improved adhesive system for bonding wood. Still more particularly, the invention is directed to unsaturated polyester resin systems suitable for bonding wood in the manufacture of wood composites, to methods of manufacturing wood composites, and to fiber-reinforced wood composites comprising such resin systems.
Glue-laminated timbers, more generally described as structures comprising parallel assemblies of face-laminated wood, have been used successfully as structural elements in buildings for over a century. The glue lamination process allows the natural flexibility of timber to be bonded into almost any rigid shape. These engineered wood structures have a superior strength-to-weight ratio and may be reinforced to meet particular design requirements in the fabrication of light, space-creating structures.
More recently, there have been developed hybrid composites wherein the wood components are combined with synthetic fibers such as glass, carbon or Kevlar, and a resin system to give greater strength, stiffness and ductility to timber. Unlike traditional steel and aluminum reinforcement, fiber-reinforced plastic (FRP) components may be engineered to match and complement the orthotropic properties of wood. Consequently, incompatibility problems between the wood and the reinforcing FRP are minimized, and the resulting hybrid materials have properties derived from each separate material. In addition, hybrid composite technology allows the use of a lower-grade wood to achieve the same, if not better, results.
Good bond strength between the wood components and the FRP reinforcement is important. Red and white oak, birch, hard maple and other high density hardwoods are difficult to bond. Moreover, if the wood product has first been treated, for example, with a preservative, or if the wood is used in wet conditions, the performance of the adhesive may be impaired. Repeated water saturation, cyclical moisture exposure followed by drying, or direct exposure to adverse weather conditions create delaminating stresses that lead to separation at the glue-line, or tension failure in the wood near the glue-line. For example, epoxy adhesives provide a bond that is as strong as the wood itself as long as the wood remains dry in service, but delaminates in wet-use conditions. Water-based wood adhesives, such as phenolics, resorcinolics, isocyanates, ureas and melamines, perform well only on wood that has not been chemically or physically altered.
Methods have been disclosed for enhancing the adhesion of a variety of adhesives to wood. For example, in U.S. Pat. No. 5,543,487, there is disclosed a resorcinol-formaldehyde derivative useful as a primer or coupling agent in bonding lumber treated with CCA preservative for service in exterior exposure conditions. The disclosure of U.S. Pat. No. 5,543,487 is incorporated herein by reference in its entirety. According to the patentees, the hydroxymethylated resorcinol coupling agent improves the adhesion of phenol-resorcinol, epoxy, and isocyanate-based adhesives to wood, including hard woods and wood treated with preservatives. The coupling agent has also been disclosed to improve the bonding of glass fiber-reinforced vinyl ester components to wood in the manufacture of hybrid wood composites.
Epoxy resins have long been used as the matrix resins for FRP reinforcement components. More recently, pultruded FRP sheets comprising glass fiber impregnated with phenol-resorcinol formaldehyde adhesives have been disclosed as reinforcement for hybrid composites. The panels are readily bonded to wood, or to one another, using the phenol-resorcinol formaldehyde (PRF) adhesives commonly used in the fabrication of laminated wood structural members. Alternative matrix resins including polyester, vinyl ester, polyimides and the like have also been suggested for this use, though adhesives for these materials that form adequate bonds with wood have not been identified.
The least expensive resins available to the FRP industry are polyester resins. Polyester resins are conveniently produced by dissolving an unsaturated, generally linear polyester in an inexpensive vinyl-type active monomer such as, for example, styrene, methyl styrene, diallyl phthalate, or the like. Cure is effected using peroxide catalysts, together with suitable promoters, or heat. Polyesters with a wide range of mechanical properties are readily available, and many are formulated to have superior resistance to wet or corrosive environments.
Although the polyester resins generally available to the art are compatible with glass fibers, most do not adhere well to wood or other lignocellulosics. Polyester adhesives have not found acceptance as adhesives for bonding wood, and the adhesives commonly employed in the manufacture of hybrid wood composites are not well suited for bonding FRP polyester reinforcement panels to each other, or to wood.
A method for improving the performance of polyesters as adhesives in bonded wood products would find wide acceptance in the art. Polyester adhesives which are capable of forming bonds to wood with adequate bond strength, and which are resistant to external exposure, would provide the industry with a wider range of materials suitable for use in the manufacture of wood composites. Pultrusions comprising fiber-reinforced polyester resins modified to have good adhesion to wood could provide low cost reinforcement for the manufacture of a wide range of hybrid composites.
The invention is directed to a method for improving the adhesion of polyester resins to wood and other lignocellulosic materials, and to modified unsaturated polyester resins having improved adhesion to wood. The method comprises modifying an unsaturated polyester resin composition with up to about 20 wt. % of a polyisocyanate.
Unsaturated polyester resins modified with a polyisocyanate are particularly useful in constructing composites from difficult-to-bond woods such as hardwoods, or softwoods and hardwoods treated with chromated copper arsenate (CCA) or other preservatives. Wood composites wherein the bonding surfaces of the wood are primed with a resorcinol-formaldehyde derivative coupling agent and then bonded with the invented modified polyester resins are particularly resistant to delamination. The invention may thus be further characterized as being directed to improved composite wood structures comprising the invented polyester resin composition and to methods for constructing such composites.